Bioremediation is a process in which microorganisms, fungi, green plants, or their enzymes are used to restore the natural environment, altered by contaminants, to its original condition. Bioremediation may be employed, for example, to attack specific soil contaminants, such as degradation of chlorinated hydrocarbons, by bacteria or to cleanup oil spills by the addition of nitrate and/or sulfate fertilisers to facilitate the decomposition of crude oil by exogenous bacteria. Additional examples of bioremediation technologies include bioventing, landfarming, bioreactor, composting, bioaugmentation, rhizofiltration, and biostimulation.
Bioremediation technologies may be generally classified as in situ or ex situ. In situ bioremediation involves treating the contaminated material at the site, while ex situ bioremediation involves the removal of the contaminated material to be treated elsewhere.
There are a number of cost/efficiency advantages to bioremediation. For example, introducing a microorganism into an environment to reduce the concentration of a contaminant in situ, is typically much less expensive than excavating followed by disposal off site, incineration, or other ex situ technologies.
However, not all contaminants are easily treated by bioremediation using microorganisms. For example, heavy metals (including, for example, cadmium and lead) are not readily absorbed or captured by organisms. The heavy metals in harvested biomass may be further concentrated by incineration or even recycled for industrial use. Motivations for controlling heavy metal concentrations in gas streams are diverse. Some of them are dangerous to health or to the environment (e.g., Hg, Cd, As, Pb, Cr), some may cause corrosion (e.g., Zn, Pb), while some are harmful in other ways (e.g., arsenic may pollute catalysts). Within the European community the elements of high concern are As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sn, and Tl, the emissions of which are regulated in waste incinerators. Some of these elements are actually necessary for humans in minute amounts (Co, Cu, Cr, Mn, Ni), while others are carcinogenic or toxic, affecting, among others, the central nervous system (Mn, Hg, Pb, As), the kidneys or liver (Hg, Pb, Cd, Cu) or skin, bones, or teeth (Ni, Cd, Cu, Cr) (Zevenhoven et al., Control of Pollutants in Flue Gases and Fuel Gases. TKK, Espoo (2001)).
Heavy metal pollution most commonly arises from the purification of metals, e.g., the smelting of copper and the preparation of nuclear fuels. Electroplating is the primary source of chromium and cadmium. Through precipitation of their compounds or by ion exchange into soils and muds, heavy metal pollutants can localize and lay dormant. Unlike organic pollutants, heavy metals do not decay and thus pose a different kind of challenge for remediation.
Once in the atmosphere, mercury is widely disseminated and can circulate for years, accounting for its wide-spread distribution. Alkali and metal processing, incineration of coal, and medical and other waste, and mining of gold and mercury contribute greatly to mercury concentrations in some areas, but atmospheric deposition is the dominant source of mercury over most of the landscape. Natural sources of atmospheric mercury include volcanoes, geologic deposits of mercury, and volatilization from the ocean. In areas where mercury has accumulated through industrial or mining activities, natural processes may bury, dilute, or erode the mercury deposits, resulting in declines in concentration. In many areas, however, mercury concentrations have actually increased because atmospheric deposition has increased. Concentrations of mercury in feathers of fish-eating seabirds from the northeastern Atlantic Ocean, for example, have steadily increased for more than a century. In certain North American sediment cores, sediments deposited have mercury concentrations about 3-5 times those found in older sediments. Some sites are considered methylmercury “hot spots” due to inadvertent human activities. Lake acidification, addition of substances like sulfur that stimulate methylation, and mobilization of mercury in soils in newly flooded reservoirs or constructed wetlands have been shown to increase the likelihood that mercury will become a problem in fish. Therefore, it would be advantageous to develop novel compositions and methods for efficiently binding at least one metal atom.